The spot on the top of the head where hair grows in a spiraling pattern is a feature nearly every person shares. This circular patch is the organizing center for the hair on the scalp, dictating the direction in which the strands lie as they grow outward. Though often referred to as a swirl, this anatomical pattern is an inherited trait that begins forming long before birth. Understanding this common feature requires looking at the biological mechanisms that establish the hair’s orientation on the head.
Naming the Swirl: What is a Hair Whorl?
The precise term for the spiraling hair pattern is a hair whorl, also known as a vortex, crown, or by the scientific term trichoglyph. A hair whorl is a patch of hair follicles that align to grow in a circular direction around a visible central point, typically located at the parietal region, or crown, of the head.
The whorl serves as the anchor and primary reference point for the entire hair coat. Hair strands extending away from this center follow its established trajectory, which is why the hair resists styling in the opposite direction. The presence of a whorl is a universal mammalian trait, most prominent on the human scalp.
How Hair Whorls Form During Development
The formation of the hair whorl occurs very early in fetal development. Hair follicles begin to form on the scalp around the 10th week of gestation, with the whorl pattern being permanently set by about the 16th to 18th week of development.
The sloping angle of the hair follicles is determined by the direction of stretch and tension on the fetal scalp skin. This tension is created by the rapid growth and maturation of the underlying fetal brain during this period. The circular pattern of the whorl is the point where opposing growth vectors meet, causing the follicles to align in a radial, spiral disposition around a central axis. Once established in the womb, the hair whorl remains stable throughout an individual’s life.
Variations in Direction and Quantity
Hair whorls are characterized by their direction of rotation, which can be either clockwise (CW) or counter-clockwise (CCW) when viewed from above. In most human populations, the clockwise rotation is the more frequent pattern. Estimates for the prevalence of a clockwise whorl range widely, from approximately 51% in some East Asian populations to as high as 94% in US newborns.
A single whorl on the crown is the most common presentation, but variations in quantity also occur. A majority of individuals possess only one hair whorl, which is the standard pattern. However, a minority of people have multiple whorls, such as a double or triple crown. Double whorls are observed in a small percentage of the population, generally appearing in about 1.5% to 5.4% of individuals.
Multiple whorls can cause hair to be challenging to manage, as the adjacent swirls pull the hair in conflicting directions. Rarely, some individuals may exhibit a diffuse or irregular pattern where a clear, centralized whorl is not distinctly visible. The presence of atypical patterns has been noted in studies concerning early brain developmental disorders, highlighting the whorl’s connection to embryological processes.
The Genetics and Handedness Connection
The direction of a person’s hair whorl is determined by their genetic makeup, but the inheritance pattern is more complex than previously thought. Early theories suggested that a single gene controlled whorl direction, but modern genetic research has largely disproven this simple Mendelian model. Whorl orientation is a polygenic trait, meaning it is influenced by the cumulative effect of multiple genes.
The direction of the whorl has also been an area of research concerning the link to handedness, or functional laterality. Some older studies proposed a connection, suggesting that a counter-clockwise whorl was more common in non-right-handed individuals. For example, one study found that approximately 45% of left-handed or ambidextrous people had a counter-clockwise whorl.
However, subsequent research has yielded conflicting results, with many studies finding no statistically significant association between hair whorl direction and handedness. Current scientific consensus treats the strict connection between whorl direction and a person’s handedness as largely unproven, emphasizing that the pattern is primarily a result of complex genetic factors influencing early development.